1. Field of the Invention
The invention relates to a device and a method for wavefront measurement of an optical system, in particular using an interferometric measurement technique.
2. Description of the Related Art
Such devices and methods are used, in particular, to determine the imaging quality of high accuracy imaging optics. An important application is the high accuracy measurement of the imaging behaviour of projection objectives in microlithography projection exposure machines. As an alternative to the use of a separate measuring site, it is possible in this case to provide to undertake the wavefront measurement of the objective in situ, that is to say in its installed state in the exposure machine. The measurement device is then integrated for this purpose in the exposure machine. The measurement of the objective is preferably performed at an operating wavelength, that is to say at that wavelength used by the exposure ma-chine in exposure mode. Such a measuring device is therefore also de-noted as an operational interferometer (OI). In a narrower sense, this term is used, in particular, for such measuring devices operating at operational wavelengths and with the aid of lateral shearing interferometry.
Such an OI is disclosed, for example, in the Laid-Open Patent Application DE 101 09 929 A1 in an implementation denoted as standard OI (S-OI). For the purpose of wavelength measurement of the objective, devices of this type of standard OI comprise an object-side mask structure element which is preferably to be arranged in or near an object plane of the objective, an image-side diffraction structure element preferably to be arranged in or near an image plane of the objective, a detector, for example a CCD camera, in the beam path downstream of the diffraction structure element, and a detector-side imaging optical system, typically with a microscope objective, between the diffraction structure element and detector. The diffraction structure element typically has a diffraction grating structure which is periodic in one or more directions, and the mask structure element functions as a so-called coherence mask and has for this purpose a suitable mask structure, which is mostly likewise periodic. The detector-side imaging optics images the diffraction structure, or the mask structure imaged thereon by the objective, into the far field, and thus images a pupil of the objective onto the detector.
Used as an alternative to the standard OI is a so-called compact OI (C-OI) which operates without the detector-side imaging optics and uses its detector to pick up the generated wavefront interference pattern in the quasi-far field. For this purpose, the detector surface is placed at a short spacing downstream of the diffraction structure element, or the radiation coming from this element is passed on to the camera surface with the aid of a so-called face plate, of which the entrance surface is placed at a short spacing downstream of the diffraction structure element.
In both the variants of standard OI (S-OI) and compact OI (C-OI), the OI does not directly detect the wavefront coming from the measuring optical system, but detects the first spatial derivatives thereof. The variation thereof, that is to say specifically the magnitude of the second partial spatial derivatives of the wavefront, determines and limits the measurement range, that is to say the dynamic range, in which the measuring device can be used. This is influenced substantially by the aberrations of the measuring optical system and, in the case of the shearing interferometry technique, by the so-called shearing distance. This can lead to a severe limitation of the measurement range, specifically when measuring optical systems in the unadjusted state, or when measuring system parts or modules of optical systems having relatively large aberrations, that is to say the phase modulation of the wavefront to be detected exceeds a certain upper limit such that the interference pattern can no longer be detected by the detector with the desired resolution over the entire active detector surface if no counter measures are taken.
It is true that consideration is given as counter measures to increasing the spatial resolution of the detector or the number of detector pixels, for example of a CCD camera, and to reducing the shearing distance in the lateral shearing interferometry technique by selecting larger period lengths of the diffraction/mask structures. However, the detector resolution is limited by the minimum size of detector pixels, and the selection of a smaller shearing distance throughout the entire detection area, that is to say the entire detected cross section of the radiation measuring the optical system, leads in cases with very irregular variation in the wavefront to the fact that the signal-to-noise ratio becomes very small for a majority of the detector pixels, and it is therefore only a small portion of the detector pixels which make an effective contribution to the wavefront measurement with good reproducibility.
Whereas, owing to the detector-side imaging optics, the S-OI images the interference pattern into the far field in a sinusoidally corrected fashion, that is to say aplanatically, onto the detector surface, in the case of the C-OI the interference pattern is imaged onto the detector surface into a plane virtually close to the far field owing to spreading in free space. In the case of a measuring optical imaging system such as a microlithography projection lens, this means that the first spatial derivative of the wavefront in a pupil of the imaging system is substantially undistorted with the S-OI, whereas with the C-OI it is already in principle not imaged in a sinusoidally corrected fashion and therefore is imaged with a corresponding distortion error. Depending on the detection system used, this can also be affected by a certain, slight distortion error. Since the wavefront measurement typically includes the measure of using the detected interference pattern to deduce the wavefront characteristic in the measured optical system and, in particular, in a pupil plane of a measured optical imaging system, in order to determine the beam guidance quality or imaging quality of the optical system, there is a need for measures which give suitable consideration to distortion errors.
In this context, Patent Specification U.S. Pat. No. 6,650,399 B2 discloses an interferometric pinhole measurement technique of calibrating a distortion error by calculating a corresponding distortion transformation by means of a so-called focal stepping, that is to say by means of a sequence of measurement operations in various axial positions of the pinhole and detector, and thus various focal positions.
Fizeau interferometers with C optics are also in use for wavefront measurement of optical systems, but are generally incapable of very compact design and are relatively susceptible to environmental influences. Moreover, their coherent light source mostly results in so-called speckled effects.
The invention is based on the technical problem of providing a device and a method which can be used to measure optical systems and, in particular, modules or subsystems of optical systems with a relatively low out-lay in a very accurate fashion by means of a wavefront measurement technique.